1. Field of the Invention
The invention relates to the implementation of facsimile communication within a radio telecommunication system and, more particularly, to a system for varying the transmission rate for facsimile data within a cellular radio system.
2. History of the Related Art
Cellular radio communications is, perhaps, the fastest growing field in the world-wide telecommunications industry. The convenience of subscriber mobility and the ability to place and receive telephone calls regardless of where the subscriber is located has introduced a new era. The convenience of telecommunications without the restrictions of a physical telephone cable limiting the mobility of the subscriber station is a great advantage.
As cellular radio communications networks have grown in recent years, facsimile communication has become common not only between facsimile apparatuses at offices connected via wired transmission lines but also between a facsimile apparatus at a fixed station such as an office and a facsimile apparatus at a mobile station such as within an automobile, or even, between two mobile stations coupled via a radio link. In the most commonly used facsimile communication procedure specified by the CCITT Group-3 standard, a facsimile apparatus at a source terminal transmits page data signals representing a coded image data for one page of a document to be transmitted. When this has been completed the facsimile apparatus sends an MPS signal indicative of the end of the transmission of that page. In the destination facsimile terminal, an apparatus receives the page data signals sequentially and decodes the transmitted image from that signal as received. During the process of decoding, the occurrence of any transmission errors is monitored and in the event the transmission errors exceed a certain threshold value, the system discontinues transmission and instructs the source terminal to resend the document.
In the transmission of facsimile data via radio links, the system is naturally more vulnerable to noise and thus more susceptible to transmission errors during the transmission of a document. When this occurs, conventional destination facsimile apparatus interrupts the transmission and requests the operator of the source machine to resend the document. This often results in the operator having to resend the document multiple times in order to complete the transmission of the data from source facsimile machine to a destination facsimile machine located at a mobile station within a cellular radio network. One means of addressing this problem is to disable the error detection mechanism at the destination facsimile apparatus so that the entire facsimile transmission is received regardless of the quality of that transmission. However, this approach is not acceptable since there is a substantial chance that the entire received document would be unintelligible without the operator at the source facsimile machine being aware of that fact.
Another solution to this problem is that proposed in U.S. Pat. No. 5,041,917 to Koshiishi in which a mobile facsimile receiving apparatus includes not only means to detect the number of errors which have occurred during transmission but also to allow a variable number of errors at various different threshold values as a function of the type of transmission medium over which the data is sent. In the event the error rate exceeds the threshold assigned to that particular transmission medium, the originating facsimile operator is notified to resend the document. This technique also has a disadvantage in that repeated resendings of the document are required regardless of whether the particular transmission link might be able to handle the data more accurately if only the transmission rate was reduced.
In the CCITT Group-3 facsimile service currently being standardized for the American digital cellular system, the facsimile standard describes a real time exchange of facsimile image data between a source facsimile machine connected to the public switch telephone network (PSTN) and a cellular mobile station (MS). The CCITT T. 30 facsimile protocol is terminated in a cellular interworking function (IWF) and is therefore not ever seen by the mobile station. The facsimile protocol proposed for use on the RF interface link between the MS and the IWF is based upon the EIA/TIA-592 asynchronous facsimile DCE control standard promulgated by Electronic Industries Association (EIA). Under this protocol, when a mobile station receives facsimile pages over an RF interface, the data throughput capability of this RF link may become deteriorated due to various transmission related parameters such as channel interference and facsimile image data is buffered in the IWF or elsewhere in the cellular network. Such buffered data must be delivered to the MS before the buffers overflow and before commencement of the reception of the next page transmitted by the source facsimile machine. Under this protocol if all the buffered facsimile data cannot be delivered to the mobile station in time, the block of data remaining in the buffer must be discarded. The facsimile call may then either be cleared entirely or continued with a partially blank facsimile page at the destination facsimile machine.
One potential solution to the buffer overflow problem associated with the buffer memory in the interworking function of the cellular network is that of providing large buffers, i.e., buffer memories big enough to hold at least one full facsimile page. However, the timing problem is not so easily solved. Real time requirements for a cellular facsimile call limits the time allowed between the complete transmission of one facsimile page and the initiation of the transmission of the next facsimile page. The IWF must completely deliver a facsimile page to the mobile station before the transmitting fax machine can begin transmission of the next page. One proposed technique for addressing the timing problem is to use facsimile machines which include error correction mode (ECM), as described in CCITT recommendations T.4 and T.30. Such error correction mode facsimile machines are illustrated in U.S. Pat. No. 4,975,783 to Takaoka. However, error correction mode cannot be mandated for each facsimile call made within a cellular mobile network and, moreover, many existing facsimile machines do not support ECM. Renegotiation of the data rate from the source facsimile machine would appear to be a logical solution, however, how to accomplish this end remains a problem.
While the preferred embodiment of the invention is employed in a cellular radio system, it should be understood that the invention can be used in other radio systems such as personal communications systems (PCS), land mobile radio, satellite systems, etc.
The system of the present invention contemplates a system for implementing the transmission of fax data within a radio communication network in accordance with the recommended protocols which reduces the need for repeated retransmissions of the data and still enables the communication of fax data within a radio network in accordance with established specifications.